Bulletin d’avalanche

Flathead Range & Glacier NP

Flathead Avalanche Center
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Danger d’avalanche

Sat
Alpin
3 · Marqué
Limite forestière
2 · Modéré
Sous la limite
1 · Faible
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Périmé — ce bulletin a expiré last year. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Recent snowfall and wind have developed dangerous conditions, especially on wind-drifted slopes. Triggering an avalanche in steep terrain is likely where the snow surface cracks beneath you or your machine. Low-angle slopes sheltered from the wind will offer the safest and best quality snow. 

Problèmes avalancheux

  • Storm Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1–2

    Thursday's dense snowfall and moderate to strong south-southwest winds built slabs on various aspects. This resulted in multiple parties triggering thin soft slab and loose dry avalanches in the new snow in the southern Whitefish Range at middle elevations. The most recent snow generally rests on a less dense layer, forming upside-down near-surface conditions. In some locations, this week's snow was deposited on weak layers, including facets and surface hoar. Both of these conditions may make slabs more reactive than expected.

    These features will be thicker, more widespread, and most sensitive on wind-drifted slopes, especially at upper elevations where you could likely trigger a slide. Slabs should be more stubborn in sheltered terrain, where triggering an avalanche remains possible. 

    Today’s blowing snow and fresh cornices point in the direction of slab formation. A rippled snow surface, pillows, or dunes indicate the wind has been at work. Cracking or collapsing indicates you have found an unstable slab, directing you to a lower-angle slope.

    (3/13/2025)

    This is the crown of a skier-triggered soft slab avalanche in Thursday's new snow. Riders could trigger this type of slide today.

    Photo: Kaleb Retz

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Unlikely
    Taille
    2–3

    Weak layers (facets, facets + crusts, surface hoar) formed in January continue to gain strength slowly. Their distribution is isolated, and triggering a large 2-4 foot thick avalanche is unlikely. However, this week's snowfall and, more importantly, wind loading potentially add stress to these layers. 

    The chance of triggering a slide increases in wind-affected terrain where the slab depth is variable and the weak layer is closer to the surface. The edges of slabs and slopes peppered with rocks are likely trigger points. Before initiating an avalanche, you may not experience noticeable signs of instability like snowpack cracking and collapsing.

    Persistent Slabs can break much wider than expected and in surprising ways. Today's surface slides or cornice fall could trigger a Persistent Slab avalanche.

    Digging into the snow is the only way to identify if your slope contains this layer. Choosing lower-angle terrain not connected to steep slopes is the easiest way to mitigate this problem.

     

    (3/10/2025)

    Recent avalanche activity in the Flathead Range/Marion Lake region. This type of rocky wind-affected slope has a variable slab depth and more likely to trigger. 

    Photo: Michael Reavis

Avalanche Discussion

Observations on Thursday were concentrated in the southern Whitefish Range, where four parties either triggered or noted soft slab avalanches and loose dry sluffs. These all appear confined to the recent storm snow, with observers noting heavy snowfall rates and elevated wind speeds contributing to a surface that became more consolidated throughout the day.  One party mentioned that the sluffs ran a long way, another noted that propagation was larger than anticipated, while a third was surprised by the touchy conditions.  Thursday's snow fell warmer and denser than Wednesday's, resulting in slightly upside-down conditions, which may have contributed to the activity. Reports of facets and surface hoar were noted before this week's snowfall, which also could have been a factor. 

Thursday's snowfall shut off by noon or early afternoon for most locations. In sheltered locations where fresh slabs are not resting on weak layers, these features are gaining strength and will be less sensitive than yesterday. Instability will linger where these rest on facets or surface hoar. 

Today's danger remains elevated in areas with recent and ongoing wind transport. Yesterday's wind gusts reached strong or gale force strength at nearly all local mountain wind stations.  Snowfall from this week is stacking up, leaving plenty of snow available for transport. The danger may creep up to CONSIDERABLE in middle elevation locations favored by wind loading. However, we feel slopes capable of this danger rating are isolated.

Curious about recent developments in tele boots and bindings? Check out Telefestivus at Blacktail Ski Area on Saturday, March 22 and Sunday, March 23. In conjunction with the mind-freeing event, Tamarack Brewing will host a fundraiser with a raffle and film for the Friends of the Flathead Avalanche Center from 5 to 8 PM Saturday, March 23. 

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